“To suggest it represents a ‘betrayal’ grossly distorts Labour’s position and is deeply unhelpful to those seeking a solution to an an issue that is reaching crisis proportions,” he told the Guardian.

McCluskey met around two dozen MPs in the House of Commons on Tuesday, where he voiced fears about the impact that backing a second Brexit vote would have on Labour.

The private intervention comes at a fractious time for the party, as the leadership agonises over what its tactics should be if Theresa May loses a vote on the Brexit deal in parliament.

McCluskey is an influential figure in the party and several of Corbyn’s most trusted staff are former Unite members and close to the union leader, including the party’s general secretary, Jennie Formby, the adviser Andrew Murray and the chief of staff, Karie Murphy.'